1. Field of the Invention
The present invention relates to a highly efficient wafer-scale microelectronic process for the fabrication of spectral filters, microoptics, optical waveguide arrays and their aligned attachment to optoelectronic semiconductor imaging devices, integrated photonic devices, image displays, optical fiber interconnection, optical backplanes, memory devices, and spectrochemical or biomedical analysis devices.
2. Background Art
Synthetic reconstruction of color images in solid-state analog or digital video cameras is conventionally performed through a combination of an array of optical microlens and spectral filter structures and integrated circuit amplifier automatic gain control operations following a prescribed sequence of calibrations in an algorithm. Fabrication of a planar array of microlenses is conventionally performed by application of a photoresist on a topmost layer of planarized film formed over red, green, blue color filters. By successive processing steps of patterning, developing, etching, followed by thermal reflow, the resist forms approximate plano-convex or hemispherical microlenses. The rheologic properties of the resist will determine the radius of curvature of the microlens elements in the planar array. Coupled with the resist's index of refraction, the resulting microlens array will have a focal length and light-collection properties which may depart from desired optimum performance, including poor control of the fill-factor of the photodiodes in an array comprising the pixel plane. Optical design of the lens shape and refractive index is extremely limited by the necessity to use photoimageable materials with restricted thermal reflow characteristics.
It is difficult to achieve long focal length high radius of curvature and high refractive index microlens arrays in a single array-plane using conventional microlens forming and fabrication processes. U.S. Pat. No. 6,482,669 B1 summarizes a number of the drawbacks of known solutions in the Prior Art. It is further noted and particularly pointed out that the present invention enables high-volume manufacturing of aspheric microlens arrays. In addition to the foregoing description of fabricating semiconductor color imagers for digital cameras, microlens arrays are also widely employed for high-resolution display monitors and for the coupling of optical waveguides in optical backplanes and optical fibers used in optical communications networks. Electrically addressable lens elements made of various liquid crystal materials are also used in lens assemblies with variable focal length and variable depth of field, or to adjust the image position to accommodate different viewing conditions. These active lens elements are on the order of tens of microns in thickness and can be switched at speeds greater than 85 MHz, enabling full spectrum color imaging without noticeable flicker.
FIG. 1A exhibits the Prior Art process 100 for the formation of a microlens array: a planar film of a photoimageable material such as a photoresist is photolithographically patterned such that exposure to actinic radiation and subsequent development of the photoresist forms a two-dimensional array of mesas which can be thermally reflowed (melted) into planoconvex microlenses under surface tension forces. An exploded assembly view is shown in 110, indicating the relative position and alignment of the microlens array elements to an underlying array of red, green, blue color filters and further underlying array of semiconductor photodetectors. By electronically amplifying and combining the outputs of the red, green and blue signals to comprise a unit of image or a picture element termed a pixel, color image formation is achieved. FIG. 1B is an isometric view showing the detailed semiconductor cross-section of the mesa-patterned photoresist 120 before reflow and the resulting planoconvex lens 130 after reflow. Topographical variations caused by the process of integrating color filters into the semiconductor, as shown in FIG. 1B, are a common problem in the Prior Art and typically require additional processing steps for adding a planarizing layer. The focal length required of the microlens elements is the vertical distance projected down to the photodetector array plane.
As the diameter of the approximately hemispherical microlens is reduced to accommodate increasing imager resolution and pixel density, the precursor photoresist film thickness scales down and the thermal reflow process of the prior art microlens formation process becomes limiting; the radius of curvature and refractive index of the reflowed lens cannot achieve the focal length requirement without significant cross-sectional thinning of the semiconductor device structure. FIG. 1C illustrates the case of collimated incident light 140 collected by planoconvex lens 150 converging a cone of light 170 passing through color filter 160 to focal plane 180 at photodetector 190. Optically generated cross-talk may result for off-axis incident image light, in spite of measures incorporating metal light shields formed between the color filters, when the optical properties of the microlens are limited by the thermal reflow process of the Prior Art, as demonstrated in FIG. 1D.